Why Do Some Animals Walk on Their Hind Legs?

A biped is any animal that uses two feet to move around. Some are really fast, some jump really far, some come from a long line of horrific beasts. But are two legs really better than four? That depends on who you ask!

Bipedalism isn’t a question of superiority; it’s about energy and evolution. Think of birds for a moment. When they’re not in flight, they’re hopping, strutting, or running around on two legs. Do you know why?

Have you ever heard of the Eoraptor? This small, two-legged dinosaur stalked the Earth roughly 230 million years ago and is suspected to be the common ancestor of all dinosaurs. This explains why so many dinosaurs were bipeds, and since birds evolved from dinosaurs, it explains why they use two only feet as well.

As for mammals, there aren’t very many that are truly bipedal. Kangaroos evolved to hop on two legs about 45 million years ago, but when moving at a slower pace, they’re back on all fours. Lizards and even cockroaches are the same way.

The world’s fastest lizard, the spiny-tailed iguana, hits its top speed on two legs instead of four.

But this doesn’t mean that bipedalism is faster. While the spiny-tailed iguana can run as fast as 34.6 km/h (21.5 mph), the four-legged cheetah triple laps it, at speeds over 100 km/h (62 mph). That is, in a sprint. Over long distances, bipeds are proven to be faster runners, and that’s because two legs take up less energy.

In a recent study, scientists put humans and chimpanzees on treadmills and compared their energy consumption. The results showed that humans used 75% less energy walking upright than chimps used while walking on all fours.

While chimps are humans’ closest modern-day relative, we diverged further back than you’d think. About 4 to 7 million years ago, human leg bones grew longer and thicker, while our hip structures adapted to accommodate bipedalism. This evolutionary shift not only conserved energy, but freed human hands to make tools, gather food, and hoist trophies!

You can talk the talk, and walk the walk, but just remember, you can’t sprint as fast as a car, or jump 25 feet at once.